Review of Femtosecond-Laser-Inscribed Fiber Bragg Gratings:Fabrication Technologies and Sensing Applications

Fiber Bragg grating(FBG)is the most widely used optical fiber sensor due to its compact size,high sensitivity,and easiness for multiplexing.Conventional FBGs fabricated by using an ultraviolet(UV)laser phase-mask method require the sensitization of the optical fiber and could not be used at high temperatures.Recently,the fabrication of FBGs by using a femtosecond laser has attracted extensive interests due to its excellent flexibility in creating FBGs array or special FBGs with complex spectra.The femtosecond laser could also be used for inscribing various FBGs on almost all fiber types,even fibers without any photosensitivity.Such femtosecond-laser-induced FBGs exhibit excellent thermal stability,which is suitable for sensing in harsh environment.In this review,we present the historical developments and recent advances in the fabrication technologies and sensing applications of femtosecond-laser-inscribed FBGs.Firstly,the mechanism of femtosecond-laser-induced material modification is introduced.And then,three different fabrication tech no logies,i.e.,femtosecond laser phase mask technology,femtosecond laser holographic interferometry,and femtosecond laser direct writing technology,are discussed.Finally,the advances in high-temperature sensing applications and vector bending sensing applications of various femtosecond-laser-inscribed FBGs are summarized.Such femtosecond-laser-inscribed FBGs are promising in many industrial areas,such as aerospace vehicles,nuclear plants,oil and gas explorations,and advanced robotics in harsh environments.

Effects of Shuanghuanglian oral liquids on patients with COVID-19:a randomized,open-label,parallel-controlled,multicenter clinical trial

We conducted a randomized,open-label,parallel-controlled,multicenter trial on the use of Shuanghuanglian(SHL),a traditional Chinese patent medicine,in treating cases of COVID-19.A total of 176 patients received SHL by three doses(56 in low dose,61 in middle dose,and 59 in high dose)in addition to standard care.The control group was composed of 59 patients who received standard therapy alone.Treatment with SHL was not associated with a difference from standard care in the time to disease recovery.Patients with 14-day SHL treatment had significantly higher rate in negative conversion of SARS-CoV-2 in nucleic acid swab tests than the patients from the control group(93.4%vs.73.9%,P=0.006).Analysis of chest computed tomography images showed that treatment with high-dose SHL significantly promoted absorption of inflammatory focus of pneumonia,which was evaluated by density reduction of inflammatory focus from baseline,at day 7(mean difference(95%CI),−46.39(−86.83 to−5.94)HU;P=0.025)and day 14(mean difference(95%CI),−74.21(−133.35 to−15.08)HU;P=0.014).No serious adverse events occurred in the SHL groups.This study illustrated that SHL in combination with standard care was safe and partially effective for the treatment of COVID-19.

Single-mode helical Bragg grating waveguide created in a multimode coreless fiber by femtosecond laser direct writing

We demonstrate the fabrication of single-mode helical Bragg grating waveguides(HBGWs)in a multimode core-less fiber by using a femtosecond laser direct writing technique.This approach provides a single-step method for creating Bragg grating waveguides.Specifically,the unique helical structure in such an HBGW serves as a depressed cladding waveguide and also generates strong Bragg resonance due to its periodicity.Effects of pulse energy,helical diameter,and helical pitch used for fabricating HBGWs were studied,and a single-mode HBGW with a narrow bandwidth of 0.43 nm and a Bragg wavelength of 1546.50 nm was achieved by using appropriate parameters,including a diameter of 10μm and a helical pitch of 1.07μm.The measured cross-sectional refractive index profile indicates that a depressed cladding waveguide has been created in this single-mode HBGW.Moreover,five single-mode HBGWs with various Bragg wavelengths were successfully fabricated by controlling the helical pitch,and this technique could be used for achieving a wavelength-division-multiplexed HBGW array.Then,the temperature and strain responses of the fabricated single-mode HBGW were tested,exhibiting a temperature sensitivity of 11.65 pm/℃ and a strain sensitivity of 1.29 pm/με,respectively.In addition,the thermal stability of the single-mode HBGW was also studied by annealing at a high temperature of 700℃ for 15 h.The degeneration of the single-mode waveguide into a multimode waveguide was observed during the isothermal annealing process,and the peak reflection and the Bragg wavelength of the fundamental mode exhibited a decrease of∼7 dB and a“blue”shift of 0.36 nm.Hence,such a femtosecond laser directly written single-mode HBGW could be used in many applications,such as sapphire fiber sensors,photonic integrated circuits,and monolithic waveguide lasers.